The objectives of this research are to analyze the compositions of solutions within carious lesions as they are affected by demineralizing and remineralizing conditions outside of the tooth. Thin sections are cut from teeth, the cut surfaces are sealed, small wells are drilled into the enamel within the carious lesions, and the sections are placed into various demineralizing or remineralizing solutions. Work to date has been concentrated on development of techniques for preparation of sections, analysis for calcium and phosphate, and measurements of pH, specific ion activities, and Galvani potentials, as required by the extremely small volumes and small specimens that are available. These techniques have now been developed, and the major emphasis of the coming research will be on following changes within the enamel lesion as determined by various independent factors such as composition and degree of saturation of the outside solution, permselective character of the surface of the tooth, and the effects of fluoride.